I created this fiddle (http://jsfiddle.net/9Qzza/16/) but jsfiddle struggles and the dialog is almost unusable in Firefox Aurora, Chrome(Version 28.0.1500.71) and Chromium.
The issue crops up when a modal draggable dialog is appended to an iframe body, thus using jQuery from a different window. The console error only pops up in firefox. There are other issues in Chrome and Chromium, however that is a different matter.
The dialog settings need to include to replicate:
When you try to drag the dialog box, the error pops up in the console.
There is a selection in the _createOverlay function that takes no care to ensure scope safety or to make a relative DOM query.
It looks like this
Since $ is not in the same scope as the dialog, the context needs to be passed into the query. Like this
var elementContext = that.element.ownerDocument;
$(".ui-dialog:visible:last .ui-dialog-content", elementContext)
The error in the firefox console is:
TypeError: $(...).data(...) is null
I have created a patch in my jquery ui fork.